What is a patron God ?

Imagination

It's largely a pagan practice wherein one of the deities in the pantheon is taken as the personal deity of the household and family. Monotheistic adoption of the practice can be seen in islam - allah started as just another deity in the Arabian pagan pantheon but was also the household god of Mohammed's family. Another example, though on a larger scale, can be seen in the adoption by the early Israelites of the Canaanite deity yah (later yahweh) as the personal deity of a tribal group.

Nothing.

fake

The Wiz has the right idea, but of course there are (in some religions) also personal patron deities. That is: it is not only cities and large groups that have patron deities, but also individuals and families, and in some cultures individuals and families *typically* have patron deities:
- personal gods
- family gods
- household gods

A patron deity might be unique to the individual or group or location or pasttime, or it might be a widely-worshiped deity that is believed to have taken a special interest in a particular individual or group or location or pasttime.

It's a god that likes tequila.